Influenza Virus Types
Influenza viruses are designated as type A, B, or C. Influenza A and influenza B viruses are responsible for most outbreaks of the flu. Influenza A viruses usually cause more extensive and severe outbreaks. While type B viruses affect only humans, influenza A viruses affect many different species (e.g., humans, birds, pigs, horses). Influenza C viruses result in mild respiratory illnesses and are not believed to cause flu outbreaks.
Influenza A viruses are further subdivided according to proteins found on the on the surface of the virus (e.g., hemagglutinin [HA] protein, neuraminidase [NA] protein). Subtypes of flu viruses include A(H3N2), A(H1N1)called swine flu, and A(H5N1)—called bird flu, or avian flu.
Wild birds are believed to be the source of influenza A infection in all other animals. Changes in influenza A and influenza B viruses can result in viruses that humans have little resistance to and that can be easily transmitted from person to person. These changes occur in two different ways: antigenic drifts and antigenic shifts.
Antigenic drifts are small changes that flu viruses undergo continuously over time. Influenza A and influenza B viruses undergo antigenic shifts. These changes in the virus result in an inability of the immune system to "recognize" the flu virus and provide protection against it. Antigenic drifts in flu viruses are the reason that a person can be infected with the flu more than once.
Antigenic shifts are major changes that occur quickly, resulting in a new virus that humans have little protection against. Only influenza A viruses undergo antigenic shifts. These changes are responsible for pandemic flu, which is different than a seasonal outbreak of flu. Seasonal outbreaks are caused by the spread of flu viruses that have already circulated among people, and pandemic flu develops as a result of a new subtype of the flu virus or a subtype that has not circulated among people.
In 1957 and 1968, worldwide pandemics occurred as a result of influenza viruses that contained a combination of a human virus and a bird (avian) virus. The World Health Organization (WHO) has developed a plan that outlines the stages of a flu pandemic and recommendations for before and during a worldwide outbreak.
According to this outline, phases of a pandemic include the interpandemic period, the pandemic alert period, and the pandemic period. The interpandemic period includes two phases. During phase 1, no new influenza subtype has been detected in people and the risk for human infection from an animal virus is low. In phase 2, there has not been a new influenza subtype detected in people, but there is a substantial risk for human infection with an animal influenza virus.
There are three phases in the pandemic alert period. Phase 3 involves human infection with a new influenza virus subtype, but no or very little person-to-person spread of infection. In phase 4, person-to-person spread of the virus occurs, but it is limited to local small clusters. Phase 5 involves large clusters of limited person-to-person spread of the virus. This phase indicates that the virus is becoming better adapted to human transmission and signifies a substantial pandemic risk.
The pandemic period includes phase 6, which is increasing and sustained transmission of the new influenza virus in the general population. Flu pandemics last longer than most public health crises and often include "waves" of increased infection. Health care workers and first responders are at increased risk for flu exposure during a pandemic.